1. Technical Field
This invention relates to communication networks and more particularly, to a communication network that switches and transports information signals in the optical domain.
2. Description of the Prior Art
Optical transmission systems are becoming increasingly prevalent in modern telecommunication networks. They offer the advantages of a low cost high bandwidth transmission medium requiring amplification at relatively long spacing and support a very large number of simultaneous communications over each of the optical links that constitute the transmission medium. Voice, data, and video are communicated as digital signals over optic fibers to minimize noise and distortion of these signals over long distance transmissions.
Sources of optical signals and destinations for optical signals are interconnected via the optical links to intermediate photonic switches which time multiplex communication signals provided on these optical links. Each of these optical links comprises a pair of unidirectional transmission optical fibers connecting the sources and destinations with the photonic switches.
In order for the time multiplexed communication signals to be switched between optical links, it is necessary that the optical signals on the optical fibers either be converted to electrical form, switched in the electrical domain and then reconverted to optical signals, or be directly switchable from one optical fiber pair to another. The former is undesirable because it requires electrical signals corresponding to the optical signals to be generated and switched. This conversion process is inefficient because the inherently limited switching speeds in the electrical domain cannot provide full compatibility with the increased transmission speed of optical transmission mediums.
Photonic switches permit the direct switching of optical fibers from one optical fiber pair to another, albeit with some penalty in performance. It is well known that optical fibers, such as the single-mode type, widely employed in optical transmission systems do not maintain a polarization condition as initially provided thereto over a very long distance. Rather, the light waves that arrive at a destination usually do so in an arbitrary polarization condition. This polarization condition is determined by the polarization of two radiation components, TE and TM, in the light waves.
The TE component is known as the transverse electrical component of which the diagrammatic representation is a vector perpendicular to the direction of propagation of the radiation. The TM component is known as the transverse magnetic component of which the diagrammatic representation is a vector which is perpendicular to the direction of propagation of the radiation and orthogonal to the TE component. The coupling length in a single-mode fiber is different for each of the components TE and TM. Also the phase mismatch induced by the refractive index change in the fiber under the influence of an electric field depends to a very high degree on the initial polarization condition of the light waves which are propagated into the fiber. The TE and TM components of a light wave are described in SINGLE-MODE FIBER OPTICS: Principles and Applications, by Luc. B. Jeunhomme, Marcel Dekker, Inc., New York, 1983, pages 1 through 3.
In the operation of a photonic switch in an optical transmission system, all signals remain optical within the switch, and are not converted to electronic signals for the switching function. The switching function may be performed by either a polarization-independent photonic switch or a polarization-dependent photonic switch, with control of the switching function being provided by electrical control signals. The polarization-independent photonic switch will switch both radiation components TE and TM in a light wave polarized in an arbitrary polarization condition and route these components of the light wave to a proper output fiber. The polarization-dependent photonic switch, however, only switches the radiation component TM in a light wave polarized in an arbitrary condition.
Although the polarization-independent switch may switch both the TE and TM components of a light wave in an arbitrary polarized condition, it has the disadvantage of requiring a higher operating voltage than the polarization-dependent photonic switch. Moreover, the level of performance in each individual switch element in the polarization-independent photonic switch is inferior to that of the individual switch elements in the polarization-dependent photonic switch.
The limitation associated with the polarization-dependent photonic switch is that it only switches the TM component of a light wave polarized in an arbitrary condition. The TE component entering the switch will not be switched by the electrical control signals, and thus will not be routed to the proper output fiber. As a result, the polarization of the light wave entering the switch from the optical input fibers must be adjusted to the proper polarization by some type of polarization adjuster in order to use this polarization-dependent photonic switching device.
While switching of optical signals from one fiber to another may be achieved with the above described arrangements, it is nevertheless desirable to be able to switch optical signals from one optical fiber to another without first converting the optical signals into electrical signals, or without the disadvantages or limitations associated with polarization-independent and polarization-dependent photonic switches as they are currently employed in the art.